We have a service trailer, its just a utility body with a fuel tank in it, oil, grease and various parts and tools in the boxes. Have had it for probably 20 years now, actually in the process of building a different one. 

Both are single wheel, I wish I could find a dually one, but for what we need the single is fine, will put 14ply tires and add some springs to the pack, may look at Timbrens too. New one will have a custom built square tank with baffles, somewhere around 300-350 gal, wanting to keep the COG lower than the current one. Old one just has a regular 300 round barrel mounted in it, has worked fine and no real issues, can get a little sloshy 1/2 full, could use a couple extra springs in the pack and Im sure it needs shocks.

New one will have electric over hydraulic brakes, using the stock Ford system on the cutoff, old one has no brakes, not a big problem as everything we pull it with is 3/4 ton.

For us the trailer isnt a big deal, usually have 2 pickups at the field anyway, if not, it gets unhooked and stays there until we need it in a different place, not much different than having a skid and a separate fuel trailer anyway. I have plenty of other little things that the new trailer will have, like self contained electrical with a solar charger for the battery that runs the pump, so you can just pull up and fuel without having to drag a cord out to pump fuel, and a hand wash system

I can go on, but you get the idea, ask any questions and if I get by to it today Ill get some pictures. Id love to have a tricked out service truck, but the trailer is much more practical, honestly it sits most of the year, most use is at harvest, in spring it gets mostly used as a fuel trailer as needed, all parts and other stuff go in the truck that has fertilizer and seed on it.  Usually it gets hooked to the beater pickup and stays hooked, that way we can leave it in the field overnight and not have to unhook it, use the other pickups to pull the header or run around in. 

I will say that if I lived in an area where theft is a problem, I might change my mind, the new box will have working locks, but theft isnt a problem out here